Fuse for Marshall amp...

[bluesbreaker59]

Okay, so I had my Bluesbreaker in to get the filter caps lowered and also to have the new Tung Sols put in, one of the Tung Sols was bad so it took out my fuse, and the amp tech I took it to was out of 500ma fuses, so he popped in a 250 ma fuse. He suggested getting a 500 ASAP. Right now I'm running it with the 250 will this hurt my amp? Also where is a good place to pick up a 500 ma fuse for a tube amp?

Thanks,
Jason

[John Phillips]

It definitely won't hurt anything, but it may blow when you play loud. The HT circuit is probably drawing around 75mA or more even at idle, and at full output is probably pulling an extra 200mA or so (I haven't measured one for a while, and I can't remember accurately). So even under normal operating conditions it may need around 300mA, which will obviously blow a 250mA fuse even if nothing is wrong with the amp. That's why they put in a 500 - it's the smallest common value above the expected maximum current draw.

You can get fuses from anywhere, like Radio Shack - for a HT fuse it can be either a fast-blow (F, or no letter) or time-lag (T, slo-blo, anti-surge), it doesn't matter really. There's no quality requirement either, they're all more or less the same unless you're talking about specialist high-current-breaking types or whatever, which you don't need. A normal wire-in-glass-tube fuse will do fine. Get a packet of them, they're very cheap and you'll almost certainly need one the next time it blows a tube.

In fact, a pair of spare power tubes and a packet of HT fuses will get you through about 90% of the likely problems with that amp.

[bluesbreaker59]

It definitely won't hurt anything, but it may blow when you play loud. The HT circuit is probably drawing around 75mA or more even at idle, and at full output is probably pulling an extra 200mA or so (I haven't measured one for a while, and I can't remember accurately). So even under normal operating conditions it may need around 300mA, which will obviously blow a 250mA fuse even if nothing is wrong with the amp. That's why they put in a 500 - it's the smallest common value above the expected maximum current draw.

You can get fuses from anywhere, like Radio Shack - for a HT fuse it can be either a fast-blow (F, or no letter) or time-lag (T, slo-blo, anti-surge), it doesn't matter really. There's no quality requirement either, they're all more or less the same unless you're talking about specialist high-current-breaking types or whatever, which you don't need. A normal wire-in-glass-tube fuse will do fine. Get a packet of them, they're very cheap and you'll almost certainly need one the next time it blows a tube.

In fact, a pair of spare power tubes and a packet of HT fuses will get you through about 90% of the likely problems with that amp.

I play pretty loud, usually 6-8 on the volume with the channels jumpered, so I should probably hit Radio Shack tonight.